This invention relates to misfire detection devices for internal combustion engines.
Generally the unburnt gas generated by the misfires of the internal combustion engines not only pollute the atmosphere but may react with the catalyst and, creating an abnormally high temperature, burn out the catalyst. Thus, it is necessary to detect an occurrence of misfire as early possible and to repair the malfunctioning parts.
Thus, various systems are already proposed for detecting misfires of internal combustion engines. For example, Japanese Laid-Open Patent (Kokai) No. 58-138271 proposes a device by which the misfires are detected on the basis of the variations of the rotational speed of the engine. In the case of this device, the average of the rotation periods of the engine is utilized as the reference, and the peak value of the length of period and this average is compared, an occurrence of misfire being determined when the difference between the peak and the average is above a predetermined level.
This conventional method of misfire detection, however, has the following disadvantage. Namely, the average of the lengths of rotation periods of the engine is first determined and this average is used as the reference value. A substantial time is needed for determining this average. Thus, it is difficult to detect misfires instantaneously. Further, since the misfire detection is based on the average of rotational speed, it is difficult to attain a high reliability of detection.